In an airplane turbojet, the high pressure sub assembly includes the high pressure compressor that provides air under pressure to the combustion chamber, and the high pressure turbine that turns a shaft that is itself coupled to the high pressure compressor in order to drive it. During maintenance operations, it is necessary to inspect the state of the rotary portions, in particular the rotor blades, with inspection initially being visual. This is commonly done endoscopically. The end of an endoscope is inserted into the casing of the high pressure sub assembly and the rotor is turned slowly in order to be able to identify visible defects.
For this purpose, it is conventional to have a crank handle that is engaged on a dedicated mechanical drive coupling of the engine gearbox. The engine gearbox is mechanically coupled to the shaft of the high pressure sub assembly, since the gearbox is normally driven by it. Since this drive coupling needs to enable the high pressure sub assembly to be turned slowly and by hand, a shaft line presenting a considerable stepdown ratio is needed in order to ensure that the amount of torque required is compatible with the force that can reasonably be expected of an operator. Proposals have also been made for motor-driven equipment to be connected to said drive coupling instead of a handle. In any event, other things being equal, the volume and the weight of the gearbox are increased by having stepdown gearing with the sole function of making endoscopic inspection possible, i.e. making it possible to turn the high pressure sub assembly slowly.
Simplifying the gearbox by omitting that stepdown gearing would enable the suspended mass that needs to be held in the event of high loading (blade loss) to be reduced. That would enable the weight of the suspension for the gearbox to be reduced, the maximum cross-section of the nacelle to be reduced, and consequently the specific consumption of the engine to be reduced. Constraints on covering the engine in the zone in question would also be reduced.